This invention relates to drivers for electroluminescent (EL) lamps and, in particular, to open loop control of the brightness of one or more EL lamps.
EL lamps are widely used for backlighting displays ranging is size from wristwatches and pagers to instrument panels in vehicles. The many advantages of an EL lamp include low power consumption, uniform luminosity (no glare), and adaptability to a wide variety of shapes, sizes, and colors.
An electroluminescent (EL) lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Thus, an EL lamp emits light when an AC voltage is applied and becomes brighter with increasing voltage. However, the phosphor powder "ages", i.e. the lamp dims with age, assuming that the applied AC voltage has a constant magnitude.
The lifetime of an EL lamp is defined as the time from initial turn-on until the lamp dims to half brightness, typically in excess of 1,000 hours. For some products, e.g. wristwatches or pagers, the lifetime of the lamp far exceeds the number of hours that a lamp will actually be turned on and the aging effect is of no consequence. For other products, e.g. instrument panels, an EL lamp may be turned on for a significant fraction of the lifetime of the lamp.
Since the brightness of an EL lamp depends on the magnitude of the applied voltage, a variety of techniques have been used to compensate for aging of the phosphor powder by increasing the voltage applied to the lamp as the lamp ages. The simplest control is a dimmer, with which one manually adjusts the brightness of the lamp to suit one's taste. For EL lamps powered from an AC powerline, a dimmer is simply a potentiometer in series with the lamp. Most EL lamps are driven by an inverter powered by a battery. The inverter typically includes a switching circuit that produces a series of pulses which are coupled to the lamp. The switching circuit can be self-oscillating or driven. Dimming inverters are known in the art and typically use pulse width modulation to change the voltage across the EL lamp.
Automatic brightness controls include optical feedback, in which a photocell monitors lamp brightness and varies the applied voltage to obtain a predetermined brightness, and analogue feedback, in which a lamp property, such as capacitance, is monitored to determine the applied voltage. U.S. Pat. No. 5,089,748 (Ihms) discloses a system including optical feedback for maintaining the brightness of an EL lamp. An optical sensor is expensive and the location of the sensor makes designing a display difficult because the sensor must be hidden and one must make separate connections to the sensor and to the lamp. Unless precision sensors are used, the part-to-part variability in sensors may cause unacceptable results.
Of the various manual and automatic control systems, optical feedback is generally the most effective in maintaining constant brightness but requires a large number of additional and expensive components and is subject to error from ambient light. For example, in automotive applications, ambient light can cause the lamp to dim and darkness will cause the lamp to be lit to full brightness--exactly the opposite of the desired response. In automotive applications, maximum brightness is desired for twilight and a lower brightness is preferred at night.
In many applications, different types of lamps are used, e.g. EL lamps, incandescent lamps, and cold cathode fluorescent lamps. Such lamps respond quite differently to a given voltage reduction, i.e. an EL lamp and an incandescent lamp will dim unequal amounts and a cold cathode fluorescent lamp may extinguish. It is desirable for all of the lamps in a given display to dim in equally.
The manual and the automatic control systems described above are "closed loop" systems, i.e. a first variable, voltage, is controlled in accordance with a second variable, e.g. brightness or capacitance. Control is more accurate if the second variable is either completely dependent or is strongly dependent on the first variable. An "open loop" control system is one in which a variable is simply set to a predetermined value.
In view of the foregoing, it is therefore an object of the invention to provide a low cost system for controlling the brightness of an EL lamp.
Another object of the invention is to provide an open loop control system for maintaining constant brightness in an EL lamp.
A further object of the invention is to provide an open loop control system which uses minimal additional components in a computer controlled display.
Another object of the invention is to provide an open loop control system for dimming an EL lamp to a given brightness irrespective of the life of the lamp.
A further object of the invention is to provide an open loop control system for dimming an EL lamp in unison with other types of lamps.